In drilling oil and gas wells, the drilling operator desires to obtain production information on the earth formation of interest. Such information includes the type and quality of fluid (whether liquids or gases) that is produced by the formation, as well as the flow rate and pressure of the fluid. Such information is useful in determining the commercial prospects of the well. A well that shows satisfactory production capability may be completed, while a well that shows no commercial promise is typically plugged and abandoned, with no further drilling expense incurred.
The desired information is typically obtained by drill stem testing. When the drilling extends the borehole into the formation of interest, a drill stem test of the formation maybe initiated. To change over from drilling to a drill stem test, the drill stem is removed from the borehole and the drill bit is taken off. The drill stem is lowered back into the borehole, with a packer and testing equipment at the lower end of the drill stem. The testing equipment is lowered to the formation of interest.
Conventional drill stem testing requires the drill stem and the drill bit to be pulled from the borehole to the surface. The borehole is then prepared for the drill stem test. Preparation includes lowering the testing equipment into the borehole, typically with the same drill stem that was used to drill the borehole.
When a well is drilled, the bit is lowered to the bottom of the borehole by a long string of drill pipe and collars. Weight is applied to the drill stem and the drill stem is rotated. This in turn rotates the drill bit, which drills the borehole deeper.
Drilling mud is circulated from the surface down inside of the drill stem. The mud exits the drill stem through jets in the drill bit. The mud then returns to the surface via the annulus, which is between the drill stem and the borehole walls.
During drilling, the mud serves several purposes. One purpose is to carry away the rock and other cuttings from the bottom of the borehole. Because the cuttings are continuously carried away, the cutting surfaces of the drill bit do not become fouled. Another purpose is to maintain static pressure on the bottom of the borehole. If the drill bit penetrates a formation with pressurized fluid, a well blowout could occur. The weight of the mud on the formation minimizes the risk of a blowout.
Unfortunately, this blowout prevention aspect of the drilling mud also serves to interfere with a drill stem test. If the formation of interest is exposed to the drilling mud, the static pressure of the mud may prevent the formation from producing during the test. As a result, the formation is isolated from the drilling mud during a drill stem test.
When the drill string is pulled from the borehole to the surface in preparation of a drill stem test, the drilling mud exits the drill pipe. The testing equipment is attached and lowered back into the borehole by the drill pipe. The testing equipment contains one or more valves. At least one of these valves is initially closed while the testing equipment is lowered into the borehole. Thus, the drilling mud is prevented from reentering the drill pipe because of the closed valve.
A packer is provided as part of the testing equipment. When the testing equipment has been lowered to the formation of interest, the packer is deployed against the borehole walls. The packer seals the annulus around the testing equipment and above the formation of interest. Thus, the formation of interest is isolated from the static pressure of the drilling mud. The drill stem test can now be conducted.
The actual drill stem test includes alternately opening and shutting the valves in the testing equipment. Opening the valves allows the formation to produce fluid up into the drill pipe. Closing the valves allows fluid pressure to build inside of the formation. Fluid pressure and flow are monitored as part of the test.
The actual drill stem test lasts only a few hours. However, the time to change over from drilling to testing and back to drilling takes much longer. In some instances, a full day of drilling can be lost due to a drill stem test. Because drilling rigs are typically leased by the day, this downtime results in greater expense in drilling a well.